1. Field of the Invention
The present invention relates to a method of manufacturing a substrate for forming a device, and a method of manufacturing nitride-based semiconductor laser diode.
2. Description of the Related Art
In a semiconductor laser diode with an edge-emission structure, light generated by recombination of carriers in an active layer is resonated by a mirror surface formed in a side surface of the semiconductor laser diode and is then lased.
In general, a single-crystal cleavage surface of a substrate and an epitaxial layer is used as the mirror surface. The characteristics of the mirror surface, such as roughness, angle and so on, have an effect upon the threshold current and reliability of the semiconductor laser diode.
In GaAs-based, Si-based, and InP-based semiconductor laser diodes using a homogeneous substrate of which the crystal surface coincides with the crystal surface of the epitaxial layer, a high-quality natural cleavage surface can be obtained by edge scribing and breaking.
However, when a heterogeneous substrate of which the crystal surface does not coincide with the crystal surface of the epitaxial layer is used, there are difficulties in forming the natural cleavage surface. In particular, when a GaN-based epitaxial layer is laminated on the sapphire substrate, the sapphire substrate and the crystal surface of the GaN-based epitaxial layer are grown so as to deviate from each other by 30 degrees. Therefore, it is hard to form the cleavage surface by using a general method.
That is, the formation of the mirror surface which does not coincide with the crystal direction increases the roughness of the cleavage surface such that an operational current of the device increases. When a notching and cleaving process for forming the mirror surface is applied, the cleavage surface eats into patterns. Then, a yield is reduced.
Meanwhile, a conventional nitride-based semiconductor laser diode is implemented on a c-plane GaN substrate. However, the c-plane of GaN crystal is known as a polar plane. Therefore, built-in electric fields generated by the polarization of the c-plane may have an adverse effect upon the combination of electrons and holes. As a result, the light emission efficiency of the laser diode may be reduced.